As is know, data compression methods effect division of the picture into picture blocks, a discrete cosine transform of these blocks to provide macroblocks of luminance and chrominance coefficients, quantization of these coefficients and variable-length coding.
Intra-type coding uses only the intrinsic content of the picture, according to the method described above.
Inter-type coding estimates the motion between, for example, the preceding picture and the current picture in order to provide motion compensation of the preceding picture, thus providing a predicted picture, the coding then being carried out on the difference between the current picture and the predicted picture.
The cosine transform allows removal of the spatial redundancy and motion compensation removal of the temporal redundancy.
It is known to slave the quantization step to the data rate of the coder in order to control this rate. The algorithms employed are generally targeted at obtaining the lowest possible rate for an optimum picture quality while taking into account the objective coding costs.
For a given rate, improving the picture quality entails removing the spatial and temporal redundancies. Algorithms of this type encounter a problem inherent in the type of coding, the appearance of defects referred to by the term block effects. Since the quantization step differs from one block to another, the decompressed picture may show the boundaries of the picture blocks, even more so if the compression factor is high and the discrepancies between quantization steps of successive macroblocks are therefore great.
The object of the present invention is to limit the aforementioned drawbacks by optimizing the control algorithm, that is to say by improving the picture quality for a given transmission rate.
To this end, the invention relates to a method for data compression of a video picture, comprising division of the current picture into macroblocks and quantizing of the data in each macroblock by calculating a quantization step as a function of a set rate, characterized in that the calculated quantization step is corrected as a function of the coding cost of the macroblock.
It also relates to a method for data compression of a video picture, comprising division of the current picture into macroblocks and quantizing of the data in each macroblock by calculating a quantization step as a function of a set rate, characterized in that the calculated quantization step is corrected as a function of an estimated or calculated motion of the macroblock of the current picture with respect to the matched one of a preceding or following picture.
It also relates to devices implementing the methods.
The spectator's eye is less sensitive to block effects for image zones in movement, because of the eye integration phenomenon. It is also less sensitive to block effects for non-uniform zones, since such uniformity or "regularity" in the picture exaggerates the aforementioned defects.